Non-pregnant gilts normally have a 21-day estrous cycle, and
heat usually lasts 24-48 hours. Natural progesterone
concentrations increase about 24 hours after ovulation, and
plateau between 6 and 10 days of the estrous cycle.
If developing embryos aren't present at days 12 to 14 of the
cycle, the progesterone level decreases rapidly.
Progesterone
and estrogen levels change as the gilt
progresses through the estrous cycle.
Administering
the synthetic progestagen MATRIX for 14 days in the feed will
effectively synchronize estrus in a selected group of gilts.
MATRIX effectively mimics progesterone, so when females are
fed MATRIX they will not come into heat. Because progesterone
works by suppressing LH (luteinizing hormone) secretion, the
final stages of follicular growth are blocked. When MATRIX
is withdrawn from the feed, follicle growth begins again.
Administered
early in the estrous cycle, on day 1 or 2,
MATRIX coincides with the normal endogenous pattern
of progesterone secretion.
Administered
in the middle of the estrus cycle,
days 10 to 12, MATRIX is initiated just before
naturally occurring progesterone begins to decrease.
For females near the end of their cycle, days 15 to 18, after
progesterone
has reached undetectable levels, MATRIX inhibits heat until
withdrawn 14 days later.
After
the MATRIX treatment period ends, up to 85% of cyclic gilts
will reach estrus within 4 to 9 days.
Percent
of gilts in estrus in gilts fed 15 mg of altrenogest per day
for 14 days on four commercial swine operations
(adapted from Intervet, 1999).1
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